Environmental Engineering Reference
In-Depth Information
33 kV
132 kV
11 kV
400 kV
Figure 5.1
Overhead lines at four different voltage levels. (Courtesy of Murray Thomson)
on steel towers (pylons), concrete towers or wooden poles, are highly visible in most power
systems worldwide. Figure 5.1 shows the typical appearances of overhead lines at various
voltage levels in the UK. It should be noted, however, that pole and tower constructions vary
considerably and do not always provide a clear indication of the voltage level in use. Note
also that underground cables, despite being much more expensive to install, are widely used
at the lower voltage levels to avoid overhead transmission line intrusion in areas of
habitation.
The 11 kV overhead line on wooden poles in Figure 5.1 is three-phase with no neutral.
This is called a
single circuit
. The steel tower lines are carrying two three-phase circuits. In
the 400 kV line each of the six conductors is made up of a bundle of four individual conduc-
tors. Each bundle behaves effectively as a single conductor, but the spacing helps to reduce
the line impedance and the losses and increases the line capacity slightly. The two three-phase
circuits can be operated independently, which provides operational fl exibility and greatly
increases reliability. Such an arrangement is known as a
double circuit
. Again there is no
neutral: the wire visible at the very top of the tower is an earth wire for lightning
protection.
The voltages indicated in Figure 5.1 are widely used in the UK. Other countries use slightly
different voltages, often with fewer intermediate levels, but the structure is always similar.
The terms
low voltage
(LV),
medium voltage
(MV),
high voltage
(HV) and
extra high voltage
(EHV) are often used but are avoided here because defi nitions vary considerably from country
to country.
Transformers, located in substations (Figure 5.2) or on poles, are used to transfer power
between lines and cables operating at different nominal voltages. A mature network may have
tens of thousands of transformers. Fortunately, transformers are both very reliable and
effi cient.
